The separation of carbon dioxide from gas mixtures, such as natural gas, flue gas, syngas and shale gas, is of industrial importance. The removal of carbon dioxide is necessary to improve the fuel quality of the natural gas or to use syngas. In addition, the combination of carbon dioxide and water can be corrosive to metal pipes, which makes the removal of CO2 necessary for transportation of natural gas. Also, carbon dioxide is a greenhouse gas that needs to be captured from flue gases to avoid harming the environment.
Current removal technologies, such as physical absorption solvents methanol, N-methylpyrrolidone, dimethyl ether of polyethylene glycol, and propylene carbonate, have many drawbacks. Common issues with these solvent removal systems are low operating temperatures and high operating pressures. In addition, effluent washes may be needed for solvents lost in the stream. Physical absorption solvents tend to be favored over chemical solvents when the concentration of acid gases or other impurities is very high. Unlike chemical solvents, physical absorption solvents are non-corrosive, requiring only carbon steel construction.
In recent years, it has been found that some ionic liquids are useful in the capture of carbon dioxide. For example, in WO 201217183, a process was disclosed for separating carbon dioxide from a gaseous stream through chemisorption by 1-ethyl-3-methylimidazolium (emim) or 1-propyl-3-methylimidazolium (pmim) containing ionic liquids with a carboxylate salt and the presence of guanidinium acetate or 1-butyl-3-methylimidazolium (bmim) acetate.
In addition, prior art has described the use of amine solutions combined with ionic liquids for removal of carbon dioxide and other impurities (US 2012/0063978 A1).
There are approximately four types of physical absorption solvents that are used for the purification of gas mixtures. These include dimethyl ether of polyethylene (DEPG), methanol, N-methyl-pyrrolidone (NMP) and propylene carbonate (PC).
Dimethyl ether of polyethylene glycol (DEPG) is a mixture of dimethyl ethers of polyethylene glycol (CH3O(C2H4O)nCH3 (where n is between 2 and 9) used to physically absorb H2S, CO2, and mercaptans from gas streams. Solvents containing DEPG are licensed, manufactured, or used in processes by several companies including Coastal Chemical Company (as Coastal AGR), Dow (Selexol), and UOP (Selexol process). Other process suppliers such as Clariant GmbH of Germany offer similar solvents. Clariant solvents are a family of dialkyl ethers of polyethylene glycol under the Genosorb® name. DEPG can be used for selective H2S removal which requires stripping, vacuum stripping, or a reboiler. The process can be configured to yield both a rich H2S feed to the Claus unit as well as bulk CO2 removal. Selective H2S removal with deep CO2 removal usually requires a two-stage process with two absorption and regeneration columns. H2S is selectively removed in the first column by a lean solvent that has been thoroughly stripped with steam, while CO2 is removed in the second absorber. The second stage solvent can be regenerated with air or nitrogen for deep CO2 removal, or using a series of flashes if bulk CO2 removal is required. DEPG also dehydrates the gas and removes HCN. Compared to the other solvents, DEPG has a higher viscosity which reduces mass transfer rates and tray efficiencies and increases packing or tray requirements, especially at reduced temperatures.
There are a number of methanol processes for acid gas removal including the Rectisol process (licensed by Lurgi AG) and Ifpexol® (Prosernat). The Rectisol process was the earliest commercial process based on an organic physical absorption solvent and is widely used for synthesis gas applications. The process operates at a very low temperature and is complex compared to other physical absorption solvent processes. The main application for the Rectisol process is purification of synthesis gases derived from the gasification of heavy oil and coal rather than natural gas treating applications. The two-stage Ifpexol process can be used for natural gas applications. Ifpex-1 removes condensable hydrocarbons and water, and Ifpex-2 removes acid gas. Methanol has a relatively high vapor pressure at normal process conditions, so deep refrigeration or special recovery methods are required to prevent high solvent losses. Water washing of 3 effluent streams is often used to recover the methanol. The Rectisol process typically operates below 32° F. (0° C.) and may be operated at temperatures as low as −95° F. (−70.5° C.). The process usually operates between −40° F. and −80° F. (−40° C. and −62° C.).
Solubilities of H2S and COS in methanol are higher than in DEPG. Rectisol's complex flow scheme and the need to refrigerate the solvent can be disadvantages with respect to higher capital and operating costs. The supply of refrigeration at low temperatures requires much power. However, this disadvantage can be outweighed by a considerable reduction of the solvent flow rate for CO2 removal as compared to other physical absorption solvent processes. Acid gas solubility in physical absorption solvents increases significantly as the temperature decreases. Low temperature also reduces solvent losses by lowering the vapor pressure of the methanol in the product streams. If H2S is to be removed from a gas with CO2 remaining in the treated gas, DEPG and NMP are more suitable than methanol.
The Purisol Process which uses NMP (N-methyl-2-pyrrolidone) is licensed by Lurgi AG. The flow schemes used for this solvent are similar to those used for DEPG. The process can be operated either at ambient temperature or with refrigeration down to about 5° F. (−15° C.). NMP has a relatively high vapor pressure compared to DEPG or PC, and the licensor recommends water washing of both the treated gas and the rejected acid gases for solvent recovery. Obviously, NMP cannot be used for simultaneous gas dehydration if a water wash is used. In general, NMP recovery with water is not necessary if the Purisol process is operated at subambient temperatures. NMP has been reported to have the highest selectivity of all the physical absorption solvents considered here for H2S over CO2. COS is not as soluble as H2S, but it is hydrolyzed by the NMP solvent. The Purisol process is particularly well suited to the purification of high-pressure, high CO2 synthesis gas for gas turbine integrated gasification combined cycle (IGCC) systems because of the high selectivity for H2S.
The Fluor Solvent process which uses propylene carbonate (PC) is licensed by Fluor Daniel, Inc. and has been in use since the late 1950's. PC is available as JEFFSOL® PC solvent and is particularly advantageous in treating syngas. PC has an advantage over the other solvents when little or no H2S is present and CO2 removal is important. PC has lower solubilities of the gas being purified: light hydrocarbons in natural gas and hydrogen in synthesis gas. This lower solubility results in lower recycle gas compression requirements for the gas flashed from the rich solvent at intermediate pressures, and lower hydrocarbon losses in the CO2 vent gas stream.
Ionic liquids are capable of solubilizing or reacting with polar molecules. Ionic liquids are comprised of a cation and anion and are liquid at or below the process temperature. Ionic liquids characteristically are non-flammable, non-degradable, viscous, thermally stable and have a low vapor pressure. Many of these characteristics would be solutions to the problems of current carbon dioxide removal technology. While many of the characteristics of ionic liquids are beneficial, the high viscosity of ionic liquids may be challenging. It has now been found that ionic liquids can be added to physical absorption solvents in a variety of weight percents to alleviate the viscosity issue, and improve the performance of the solvent.